A gene is a sequence of A’s, T’s, G’s, and C’s that usually provides the instructions for a single protein component of an organism.

The letters of the genetic alphabet – A, T, G, and C – are meaningless on their own, but they are combined into useful instructions in genes. Some genes carry enough information for one complete characteristic of an organism, but most characteristics result from combinations of genes. Genes are like chapters in the books that fill the library of the genome.

DNA Sequencing

The sequence of letters within a gene is like the letters in a book of instructions. Deciphering the enormously long sequence of A’s, T’s, G’s, and C’s in an organism’s genome reveals useful information. For example, finding a difference in a gene sequence that governs muscle structure raises questions. Could the difference affect health? Just as changing one letter in a word can change its meaning – for example, mice to rice to nice – so changing one DNA letter can sometimes cause illness.

Not all of the sequences in the genes of two humans are identical. For example, because your face is unique, the precise set of sequences in the large group of genes that control the shape of your face are presumably unique too. Some special parts of the DNA sequence vary from person to person with unusually
high frequency. As you will see, finding sequences in DNA samples can be used to identify individuals and help solve crimes, even when there are no eyewitnesses.